Diaphragm pumps commonly used for airless sprayers have received a great deal of design attention to alleviate various problems of wear, breakdown and expense of manufacturing. Toward that end, several devices have been developed which provide hydraulic feedback governed by the outlet pressure of the pump and which govern the pressure of the driving fluid. For example, U.S. Pat. No. 4,353,684 discloses a double poppet relief valve, one for low outlet pressure and one for high outlet pressure. As the outlet pressure increases, the low pressure poppet relief valve is biased closed causing the secondary poppet relief valve to act as the primary relief valve, it being set to open at a higher pressure value. U.S. Pat. No. 3,779,384 shows an output regulated valve which operates by decreasing the inlet fluid flow to the driving fluid chamber. In U.S. Pat. No. 3,433,161, the overflow valve is controlled by the outlet pressure of the driven fluid. An increase in the outlet pressure further biases the valve head into its seat, thus increasing the relief valve pressure point for the driving fluid. U.S. Pat. No. 3,416,453 discloses an automatic pressure limiting device with a hydraulic servo motor which includes a differential piston having opposing pressure faces responsive to the outlet pressure. This device releases excess pressure from the driving side of the diaphragm when the outlet gas pressure drops below the hydraulic pressure on the driving side. None of the above patents properly addresses the problem resulting from a blockage or temporary cessation of outlet fluid flow with the pump motor continuing to work against the diaphragm. Continual input on the driving side of the diaphragm where there is no outlet for the driven fluid results in a continual, excessively high pressure pounding against the diaphragm and against the conventional check valves. The resulting increased wear of these parts will shorten their life and tends to cause malfunctions of the pump.
Conventional pumps with no feedback such as the one disclosed in U.S. Pat. No. 3,680,981 will permit bypass of driving fluid in a stand-by mode in response to a high preset driving fluid pressure which is also equal to the driven fluid pressure. Both the inlet and outlet valves of the driven fluid side are still operating in this stand-by mode. Though operating at a reduced level, these inlet and outlet valves are being needlessly subjected to increased wear as are other components such as the eccentric bearing and the valves on the driving fluid side. Further, if the pump is shut off without unloading the preset pressure, the pump will not restart unless all pressure is released either by opening the hydraulic pressure valves or unloading the paint prime valve.
Another consequence of the excess hydraulic pounding on the driving fluid side of the diaphragm is hydraulic fluid blow-by, or seepage out of the driving fluid pressure chamber back into the sump. The use of conventional o-rings, such as those shown generally in U.S. Pat. Nos. 4,184,809 and 3,961,860, may offer temporary protection. But, seals such as these, relying in part for their effectiveness upon compression between two adjacent and mechanically joined components, are prone to failure as the two components loosen relative to one another. A more reliable solution has been to manufacture these components following strict tolerance guidelines. Although this has significantly reduced the fluid blow-by problem, it has proven to be much more costly.
What is needed is a device responsive to excess pressures in the outlet fluid flow which will correspondingly cut off the high pressure hydraulic input to the diaphragm. Also needed is an inexpensive manner of preventing fluid blow-by from the driving side of the diaphragm.